Optical tracker systems are used for tracking a rotating body such as a fan or rotor blade in order to measure the displacement thereof. Optical tracker systems are well known. Among other things, optical tracker systems are used to detect the displacements of helicopter rotors and may be embodied in portable maintenance or test equipment or in a more sophisticated health and usage monitoring system (HUMS). HUMS apparatus is mounted permanently in a helicopter and the optical tracker system associated therewith is used continuously to measure rotor blade displacement during all flight operations of the helicopter. Consequently, a greater performance is required of the optical tracker system since it must function in all normal operating environmental and flight conditions and not just the controlled conditions of a special purpose test flight.
Known optical tracker systems are described in for example U.S. Pat. No. 2,960,908 (Willits et al.), issued on Nov. 22, 1960, U.S. Pat. No. 3,002,420 (Willits et al.), issued on Oct. 3, 1961, U.S. Pat. No. 4,812,643 (Talbot), issued on Mar. 14, 1989, and European Patent Application No. 0 208 399 (Stewart Hughes Limited), priority date of May 16, 1985. The systems described in these publications include an optical sensor comprising a lens and usually at least two light sensing devices such as photodiodes. The lens defines a field of view and as a blade passes through the field of view an image of the blade is formed on the light sensors and is detectable thereby. As the blade passes through the field of view the blade image moves across the photodiodes which produce respective output signals that vary in time and represent the passage of the blade. The time variations in the output signals from the photodetectors can be used to determine the displacement of the rotor blade.
The image formed by the lens comprises the blade and the background and there must be sufficient contrast in the image between the blade and the background in order to be able to distinguish between the two. Optical tracking devices may be used in flight during the day or at night. During the day the background will usually be bright daytime sky and adequate contrast can be achieved by arranging for the portion of the blade that passes through the field of view of the sensor to be much darker than the background. At night, the background will usually be dark night sky and adequate contrast can be achieved by illuminating the blade or a reflective patch mounted to the blade as the blade passes through the field of view of the sensor.
Signals from the sensing device are input to signal processing circuitry where they are analyzed in order to determine the relative position of the blades as they rotate. The processing circuit may be a dedicated electronic circuit or it may be sophisticated computer based circuitry.
While known systems work adequately there is nevertheless a problem associated with various imperfections in the optical sensor which result in stray light arriving at the photodiodes along different optical paths within the optical sensor.
Light scattering within the optical sensor can reduce significantly the contrast between the background image and the blade image. Scattered light is created because of various imperfections in the optical sensor. A small component of light is reflected back out through the entrance of the optical sensor but the majority of the light remains in the optical sensor and must be absorbed in order to avoid the aforementioned reduction in contrast.
One way in which the reflected light can be absorbed is by way of a field or aperture stop. It is not feasible to make a field or aperture stop completely absorbent of reflected light and therefore in practice some portion of the reflected light will remain causing a reduction in the contrast of the image.
The effects of scattered light can be further minimized by ensuring the cleanliness of all optical components and blackening the inside surfaces of the optical sensor. It is also possible to compensate for the reduced contrast electronically by measuring the observed light and dark levels in the image and using this information when deducing the size or shape of the image from the signals output from the light sensors.
Another way of reducing the amount of scattered light is by shielding the lens from undesired light sources using a lens hood. However, it is difficult to achieve this for an optical sensor which is permanently mounted to the body of a helicopter as part of a HUMS apparatus. If a lens hood is mounted on the surface of the helicopter body it will project into the airflow and increase drag which is clearly undesirable. If the lens is mounted at the bottom of a cavity formed in the helicopter body there is a danger that the cavity will fill with water. In both cases there is also an icing hazard and therefore preferably the optical sensor is mounted to the helicopter so that the lens is substantially level with the outer surface of the helicopter body. As a result, during daytime flights the full light of the sun may be incident on the lens which will give rise to a high level of scattered light within the optical sensor.
The brightness of the daytime background sky may be quite low, particularly at high altitudes where the sky is much darker and bluer than near ground levels. Under such circumstances the contrast between the background and foreground blade image will be relatively low. Furthermore, under certain circumstances the level of scattered light incident upon the light sensors from illumination by the sun may equal or even exceed the light level of the background sky. Generally, the rotor blades are positioned above the optical sensor and it is not uncommon as the rotor blades rotate for them to pass between the sun and the optical sensor. Indeed, this can occur even when the field of view of the optical sensor is directed away from the sun. When the shadow of the blade passes between the sensor and the sun, the amount of scattered light entering the sensor is considerably reduced. This significant reduction in scattered light is sensed by the light sensors and converted into a signal which is output therefrom. This signal can be misinterpreted by the processing circuitry as representing the image of a blade passing through the field of view of the sensing system. The processing of the false signal will give rise to an erroneous measurement.
A detection system using photodiode detectors is described in UK Patent Application No. 2,076,606 A (The Solartron Electronic Group Ltd.), published in Dec. 2, 1981. The system is arranged to discriminate between true signals and those resulting from interference. A monostable circuit provides a time window within which true signals are detected and without which interference signals are rejected.
The present invention aims to overcome the aforementioned problems.